Generally, in the case of a thin film organic FET using organic semiconductors, organic semiconductor layers can be formed by a simple process such as a printing method, a spray method, or an ink-jet method, therefore, the cost is considerably lower than that of an FET using inorganic semiconductors. Moreover, since there is a possibility that a light and thin integrated circuit having a large area may be formed easily, the application thereof to a liquid crystal display, an organic EL display, an IC card, etc., is expected.
Recently, the mobility of carrier of the organic semiconductor is increased and those having the mobility of carrier as high as that of the amorphous silicon have been found. The research on how to put to practical use an FET using organic semiconductors having such a high mobility is extensively being carried out. Specifically, organic materials that exhibit a high mobility and are currently available include pentacene, polyalkylthiophene, etc., as a result, a great progress in the development of the organic FETs has been found.
However, even though these materials are used, such a high mobility as that of the amorphous silicon can be obtained only when the materials are molecular substances and are used in the form of a single crystal. If the entire semiconductor layer is made up of a single crystal, it is extremely difficult to manufacture a large integrated circuit at a low cost. On the other hand, organic semiconductors made of polycrystalline and amorphous polymers cannot be put to practical use because of the mobility incommensurably lower than that of a single crystal due to, for example, the loss of scattered electrons at grain boundaries. Therefore, in order to prevent a material from being brought into a polycrystalline state, in other words, to prevent occurrence of crystal defects, a considerable amount of man-hours are paid for purification of the material to reduce the concentration of impurities in an organic semiconductor layer as much as possible.
In order to solve these problems, a proposal is made in which the mobility of a sexithiophene evaporated film, which is an organic semiconductor layer, is increased by using cyanoethyl pullulan as a material for a gate insulation film in an FET structure (refer to Patent document 1).
Patent document 1: Japanese Patent No. 2984370